To be effective in a broad range of situations, a 6-character party needs to cover all of its bases - social, magic, combat, exploration, and investigation. And all character professions can be useful in any of those situations (magic's perhaps the hardest situation in which non-magickers can be useful, but it's a poorly written adventure that requires a specific adventuring profession to be present in a party).
A comparison of which professions are better, though, is unfair, as the later professions follow very different design principles to the earlier professions, and the early professions have not been retrofitted with those principles in mind. Two obvious examples are the knight and the knave, both of whom have exceptional social advantages, but only with the knave is this reflected in the rules. Similarly, for the two scholarly professions, sorcerer and priest, only the priest's scholarly lore represented by any rules.
Professions in Dragon Warriors are not balanced, they are not equivalent, nor are the skills all useful. Instead, they are based on evocative archetypes that a player may want to play, regardless of how useful or otherwise they may be during an adventure - that's the job of the GM, to sculpt an adventure that gives every character an opportunity to shine. If you have a DW party where the knight does all the fighting, the assassin does all the sneaking, the knave does all the talking, and the sorcerer just spends MP on healing the party then you might as well be playing D&D. In a nutshell, DW professions have character, as opposed to characters having a profession.
But, that little rant aside, if I was in-game and had to recruit a varied set of 6 adventuring professionals without knowing anything about the adventure on which they were being sent, and assuming they were all 1st rank, then I'd have the following:
1) Knight - every party needs a leader, almost all adventures are going to need (at least one) strong combatant (best AF in the game, and AF is perhaps the biggest factor in keeping a character alive at early ranks), and the knight's social advantages can be very useful in non-underworld settings.
2) Sorcerer - covering the next base, magic, few can hold a candle to what a sorcerer can accomplish with magic - even at first rank, a sorcerer can blast enemies with fire, heal the wounded, illuminate the darkness, deceive with illusion, and pick locks (sort of
). You can't really ask for much more from anyone!
3) Assassin - most adventures will take characters into the underworlds, where exploration skills are essential - I'm not fussed about the assassin's various combat abilities, but at first rank the assassin could have two exploration abilities, such as climb, jump, pick locks, and let's not overlook his ability to sneak, which has a plethora of adventuring applications.
So, the first half of the party was easy enough, but the next half requires a little bit more thought - remembering that we have 6 slots of 1st rank adventurers, there are three obvious comparisons to make:
4) Barbarian or Hunter
The barbarian is a faultless fighter and tracker, with good armour, high health points, and exceptional combat skills. However, the hunter brings exceptional ranged combat ability, and some stealth skills to back-up the assassin. Additionally, his foraging skills mean that any possibly reason you'd like to have a low-level elementalist around (i.e,. for the food and water spells, and possibly the ranged attacks) goes away.
5) Mystic or Warlock
We can't leave all of the magic to the sorcerer. We have 6 slots to fill, so we can have a bit of redundancy. Elementalists have very few spells and, at first level, may not be able to afford an elemental focus, which scuppers their spellcasting in their secondary elements at first rank and also means they'll only be able to cast their first level spell from their primary element once per day. Even then the low-level spells are a bit feeble. So that leaves the mystic and warlock. The mystic's chief advantage is to be able to
See Enchantment as an always-on ability. Even a sorcerer cannot detect magic until 2nd rank. However, the warlock can detect magic at first rank, heal others (combat is deadly, so you can never have too much healing, and the mystic may only heal himself), and brings some much-needed extra fighting ability to the party. So I'd go with warlock.
6) Priest or Knave
Having filled the first 5 slots and balanced our party pretty well, we have the luxury of a sixth slot, which means we can be pretty specialist with our selection - both priest and knave bring additional social benefits to those provided by having a knight in the party, but their skills each have a very different focus that makes direct comparison difficult. Priests can contribute their knowledge and language skills and knaves, by choosing starting skills that do not overlap with the assassin, can also fit very well with a party (and could bring language skills, too, like the priest). I'd be tempted to choose knave over priest, simply because the +1 per rank for Looks rolls is going to become a phenomenal advantage to the group and, as this is a 'luxury' pick, I can afford to play the long game by choosing knave.
So how have I done covering my bases? I have knight and knave for social, sorcerer and warlock for magic, knight, barbarian and hunter for combat, assassin and hunter for exploration, and assassin and knave for investigation.
Why no elementalists? Too few spells and not enough distinction to give them a role that cannot be covered by the other professions. The only reason to play an elementalist, as a player, is if you want to play an elementalist because it's an elementalist.
Of course, it's a very different exercise when looking at hiring an experienced bunch of adventurers or have a smaller party to take adventuring...